1. Field of the Invention
The present invention relates to a dispersion suitable for use as a coating formulation, particular for coating metal surfaces for can applications, and a method for coating a substrate with the dispersion.
2. Introduction
Dispersions of polymeric substances are desirable for coating applications. One particular field of coating technology is that of coating metal surfaces for use as cans. Coating of a metal surface is desirable to inhibit interaction of the metal with contents of metal cans (for example, food and beverages packaged within the can). The coating itself must provide barrier properties, be durable, and not interact with or contaminate can contents in such can coating applications. The coating must also be sufficiently flexible so as to allow coating onto a flat sheet of metal and subsequent rolling and shaping of the sheet metal into a can without damaging the coating.
Epoxy and vinyl chloride polymers (for example, polyvinyl chloride (PVC)) have been one option for can coatings. However, the industry is looking for a new generation of coatings to differentiate from epoxy and vinyl chloride polymers coatings to achieve lower film weights, reduce curing temperature requirements, increase flexibility of the coatings, and to avoid components that may have environmental concerns.
Water-borne dispersions (that is, dispersions with an aqueous continuous phase) are also increasingly available for coating substrates such as metal surfaces, primarily as spray coatings. However, water-borne dispersions are generally incompatible with coating equipment designed for use with organic-based compositions, which generally rely on roller-coating applications. Traditional organic-borne coating formulations are typically prepared by dissolving resin and other components into a organic continuous phase, the solubility of which can limit the fraction of resin that can be included in the formulation. As a result, organic phase concentration can end up especially high (greater than 70 weight-percent of the formulation), which corresponds to low efficiency and a high level of volatile organic components in the coating.
Therefore, it is desirable to identify a organic-based (organic phase continuous) dispersion that is suitable for use for coating metal substrates for use as cans and that is free of epoxy and PVC and that is not limited by the solubility of resin in the organic phase.
It is further desirable to identify a organic-based dispersion that comprises low cost non-functionalized polyolefin as a significant if not majority component of the coating solids weight in order to keep costs low relative to functionalized polyolefin dispersions. It is yet further desirable for such a organic-based dispersion to be at least 30 percent by weight solids (based on total dispersion weight) in order to minimize organic continuous phase and maximize coating coverage and coating weight. Yet more desirable is for the particle in the organic-based dispersion to have a particle size of less than 10 micrometers in order to achieve long-term stability of the dispersion.